1. Field of the Invention
The present invention relates to a back light for a liquid crystal display (LCD) device, and more particularly, to a direct type back light in which U shaped lamps for divisional driving are used. More particularly, the present invention relates to removing a bright line or a dark line occurring in a juncture area between the lamps to obtain back light uniformity.
2. Discussion of the Related Art
Recently, flat panel displays have been studied actively. Among them, an LCD device, a field emission display (FED) device, an electro-luminescence display (ELD) device, a plasma display panel (PDP) have received much attention.
It is a general tendency that the LCD device is widely used in the field of notebook PCs, desktop monitors, and liquid crystal TVs owing to its advantages of high contrast ratio, features suitable for displaying gradation or moving pictures, and low power consumption.
However, since the LCD device is a nonradiative device, an outer light source is separately required to irradiate light. Particularly, a transmission type LCD device essentially requires a separate irradiating device, i.e., a back light that emits and guides light on a rear surface of an LCD panel.
The back light may be classified into an edge type back light and a direct type back light depending on how light is transmitted.
The edge type back light includes a tube type linear light source such as a light-emitting lamp (hot cathode and cold cathode) provided on a side of an LCD panel. This edge type back light transmits light from the light-emitting lamp to an entire surface of the LCD panel using a transparent light guide panel.
The direct type back light includes a light-emitting lamp selectively provided below an LCD panel, and a diffusion sheet provided between the light-emitting lamp and the LCD panel. This direct type back light uniformly distributes light from the light-emitting lamp on an entire surface of the LCD panel after diffusing the light using the diffusion sheet.
Since the direct type back light does not require a light guide panel, it can provide a slim and lightweight size, high luminance and uniform light distribution.
Hereinafter, a related art direct type back light will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view illustrating a related art direct type back light, FIG. 2 is a plane view illustrating another related art direct type back light, FIG. 3 is a sectional view taken along line I-I′ of FIG. 2, and FIG. 4 is a photograph illustrating problems of the related art direct type back lights.
First, an LCD device based on a back light will be described.
Generally, the LCD device includes an LCD panel, a polarizing plate, a direct type back light, a case, and a bezel. The LCD panel includes a liquid crystal layer, and upper and lower substrates bonded to each other by interposing the liquid crystal layer therebetween. The polarizing plate is respectively provided on upper and lower surfaces of the LCD panel to transmit only light in one direction. The direct type back light is provided a slight distance from the LCD panel to provide light to the LCD panel. The case surrounds an outer side of the direct type back light to support the LCD panel and the direct type back light. The bezel is provided outside the case to surround a corner except an effective area for displaying images. The bezel is made of a stainless steel material.
In FIG. 1, the direct type back light employs cold cathode fluorescent lamps (CCFL) and includes a plurality of light-emitting lamps 111, an outer case 103, and optical sheets. The light-emitting lamps 111 are fitted into grooves formed at both sides of the outer case 103. The outer case 103 fixes and supports the light-emitting lamps 111. The optical sheets are arranged between the light-emitting lamps 111 and an LCD panel (not shown), and include a diffusion sheet 115, a prism sheet 116 and a protective sheet 117.
The optical sheets prevent outlines of the light-emitting lamps from being displayed on a display surface of the LCD panel and provide a light source having uniform brightness distribution. The sheets are arranged to improve a light dispersion effect.
A reflecting plate is arranged below the light-emitting lamps 111 to focus light from the light-emitting lamps 111 on the display surface of the LCD panel, thereby maximizing an efficiency of the light output by the light-emitting lamps 111.
However, the aforementioned light-emitting lamps 111 have limitations in divisional driving of the LCD device. In divisional driving of the LCD device, the entire display area is divided into several different areas, the light of the back light is increased in a display area for bright images and decreases in a display area for dark images to obtain dynamic display performance. In this case, instead of using the straight direct type light-emitting lamps of FIG. 1, U shaped lamps 211 are used, as shown in FIG. 2 to improve the efficiency of the lighting.
FIG. 2 illustrates a structure of the display area divided into eight areas. The U shaped lamps 211, as shown in FIG. 2 and FIG. 3, are provided with a phosphor 205 coated on an inner wall of a U shaped glass tube. A discharge gas of inert gas or Hg is injected into the U shaped lamps 211 and then both ends of each lamp 211 are sealed. Electrodes 208 are respectively arranged at both ends of the glass tube. A power source lead-in 209 is connected to the electrodes 208 to transfer a power source for driving the lamps. The power source lead-in 209 is coupled to an inverter 206 after being connected with a connector 207.
As illustrated in FIG. 2, the respective U shaped lamps 211 are symmetrical to each other so that their curves are positioned at the center. In this case, the light at the center may be brighter or darker depending on the distance between the facing curves of the U shaped lamps.
If the distance between the facing U shaped lamps is long, as shown in FIG. 4, a dark line A such as a dark stripe occurs at the center of the display area. For this reason, uniform light distribution is not obtained. Conversely, if the distance between the facing U shaped lamps is short, a bright line occurs at the center of the display area.
As described above, when the U shaped lamps are arranged to be symmetrical to each other to divisionally drive the LCD device, a dark line or a bright line can occur at the center of the display area, which deteriorates a picture quality of the display device.